Modulators are used within transmitters to modulate an input signal with a radio frequency (RF) signal. The modulated input signal may include voice and data, for example. One type of modulator is an in-phase/quadrature (I/Q) modulator. An I/Q modulator receives an in-phase (I) signal and a quadrature (Q) signal and modulates the I and Q signals with an RF signal.
An I/Q modulator is also known as a vector modulator, and is commonly used to support different types of modulation in a single package. Many transmitters rely on an analog implementation of an I/Q modulator. An analog I/Q modulator typically requires calibration whereas a digitally implemented I/Q modulator typically does not.
More particularly, an analog I/Q modulator suffers from imperfect carrier rejection 10 caused by DC offsets between signal paths, as illustrated in FIG. 1 for a single tone modulation. An analog I/Q modulator also suffers from imperfect sideband rejection 12 caused by gain and phase imbalance, as illustrated in FIG. 2 for a single tone modulation. These imperfections lead to gain, phase and DC offset imperfections which may degrade transmitter performance. Degraded transmitter performance includes reduced adjacent power rejection, FM distortion and AM ripple.
Imperfect carrier rejection and imperfect sideband rejection can be compensated manually or electronically. Single tone approaches may require feedback paths and analog-to-digital converters with sufficient speed to measure the feedback envelopes.
One calibration approach is disclosed in an article titled “Adaptive Compensation for Imbalance and Offset Losses in Direct Conversion Transceivers” by Cavers, IEEE Transactions on Vehicular Technology, Vol. 42, No, 4, November 1993, pp. 581-588. In a calibration phase, a continuous calibration tone is provided to the I/Q modulator for modulation, and an envelope detector receives the continuous modulated calibration tone. Sample points from the continuous modulated calibration tone are then converted from analog-to-digital for input to a calibration algorithm. A drawback of this approach is that the analog-to-digital converter as well as the feedback data needs to be fast enough to read the envelope data output from the modulator to measure the tone frequencies. This requirement results in increased cost and power consumption. Not only is proper timing alignment needed between the generated continuous calibration tone and the feedback envelope, but multiple analog-to-digital converters may be needed, which further results in increased cost and power consumption.
Another approach for calibrating an I/Q modulator in a transmitter is disclosed in U.S. Pat. No. 6,798,844. The transmitter includes an I/Q modulator and a compensator for correcting the phase and amplitude imbalance caused by the I/Q modulator. A feedback path samples the I/Q-modulated test signal to be transmitted, an analog-to-digital converter converts the signal samples taken from the test signal, a demodulator demodulates the signal samples digitally into in-phase and quadrature feedback signals, and an adapter determines the phase and amplitude imbalance caused by the I/Q modulator on the basis of the in-phase and quadrature feedback signals. The adapter then determines and provides to the compensator the correction parameters of phase and amplitude on the basis of the determined phase and amplitude imbalance.
Yet another approach to calibrate a modulator is disclosed in U.S. Pat. No. 7,092,454. Calibration parameters are provided to a calibration network so that the modulator receives a pair of predetermined sinusoidal in-phase and quadrature signals and outputs a distorted modulated signal. A processor then processes spectral parameters at first and second harmonics of a detected envelope signal of the distorted modulated signal to generate the calibration parameters for the calibration network.
A drawback of the approaches disclosed in the '844 and '454 patents is that they also require proper timing alignment between the generated calibration parameters and the feedback envelope, as well as requiring the calibration to be performed at sufficient speed to measure the feedback envelopes.